Backside illumination (BSI) image sensor chips are replacing front side illumination sensor chips for their higher efficiency in capturing photons. In the formation of the BSI image sensor chips, image sensors and logic circuits are formed on a silicon substrate of a wafer, followed by the formation of an interconnect structure on a front side of the silicon chip. The interconnect structure includes a plurality of metal layers including bottom metal layer M1 through top metal layer Mtop.
The wafer is then flipped over. A backside grinding is performed on the silicon substrate from the backside of the silicon substrate. A buffer oxide layer may be formed over the back surface of the remaining silicon substrate, and a first opening is formed to extend from the buffer oxide layer to stop at a shallow-trench isolation (STI) pad that is formed in the silicon substrate. A second opening is then formed inside the first opening to further etch the STI pad and the interlayer dielectric (ILD) that is directly under the etched portion of the STI pad, so that a metal pad in the bottom metal layer M1 is exposed. The second opening is smaller than the first opening. An aluminum copper pad is then formed in the first and the second openings and electrically coupled to the metal pad in metal layer M1. The aluminum copper pad may be used for bonding to the BSI chip.
In the above-discussed conventional formation process for forming the aluminum copper pad, two masks are required to define the patterns of the first opening and the second opening. The resulting structure is free from the electrical short between the aluminum copper pad and the silicon substrate, and is free from the electrical short between aluminum copper pads. Also, through the above-discussed processes, a clear BSI alignment mark (known as a scribe-line primary mark (SPM)) may be formed for the alignment of the subsequent formation of color filters.
In the above-discussed process steps, the formation of each of the first and the second openings requires a corresponding lithography, etching, and photo resist removal process. The manufacturing cost is thus high.